1. Field of the Invention
This invention relates to a process for the production of coatings using coating compositions containing as binder highly compatible and readily pigmentable mixtures of lacquer-grade polyisocyanates with selected polyhydroxy polyacrylates.
2. Description of the Prior Art
German Offenlegungsschrift No. 2,460,329 and British Pat. No. 1,515,868 describe a process for the production of low molecular weight acrylate resins, i.e. acrylate resins having a molecular weight of from about 500 to 2000, which are obtained by radical copolymerization in bulk or in an organic solvent or solvent mixture at temperatures in the range from 80.degree. to 120.degree. C. In conjunction with certain diisocyanates and polyisocyanates, the acrylate resins in question, which consist of from 10 to 50 parts by weight of a hydroxy alkyl ester of acrylic acid or methacrylic acid, from 0 to 80 parts by weight of styrene or derivatives thereof, from 10 to 90 parts by weight of a (meth)acrylic acid ester containing from 1 to 12 carbon atoms in the alcohol radical and from 0 to 50 parts by weight of an .alpha.,.beta.-monoolefinically unsaturated monocarboxylic or dicarboxylic acid containing from 3 to 5 carbon atoms, give lacquers which are suitable for two-component lacquering. However, one disadvantage of the resins in question is that, in some cases, they show poor compatibility with a number of polyisocyanates which may be reflected both in clouding of the mixed solution when the two components of the lacquer are combined with one another and also in clouding of the dried, hardened lacquer film. In most cases, the resins also show relatively poor compatibility with a number of cellulose derivatives of the type often added to the lacquers as levelling agents or as an additional lacquer component.
Another serious disadvantage of the lacquer mixtures in question lies in their generally poor pigment wetability. Accordingly, pigmented, hardened lacquer films on this basis generally show very little gloss, if any, and lack uniformity in their appearance on account of the irregular distribution of the pigment particles.
U.S. Pat. Nos. 3,002,959 and 3,375,227 and German Auslegeschrift No. 1,038,754 also describe polyacrylate resins containing hydroxyl groups and processes for their production. The polyacrylate resins in question may be hardened with crosslinking agents, including diisocyanates, to form lacquer layers. There are no concrete references in any of these patents as to how the difficulties referred to above can be obviated, particularly where higher than difunctional polyisocyanates containing biuret, urethane or isocyanurate groups are used as crosslinking agents.
Accordingly, the object of the present invention is to provide a new process for the production of coatings using binders based on lacquer-grade polyisocyanates and polyacrylate resins containing hydroxyl groups which are not attended by any of the disadvantages referred to above. This object is achieved by the process according to the invention which is described in detail in the following.